The invention relates to polymer compositions with high vinyl end segments and methods for their preparation.
Polymers and copolymers of conjugated dienes such as polybutadiene, polyisoprene, and styrene-butadiene rubbers possess physical properties which make them suitable for many important applications such as synthetic rubbers and as additives to other polymeric systems such as, for example, high impact polystyrene (HIPS). Moreover, HIPS can be manufactured by polymerization of styrene in the presence of 5-10% dissolved polybutadiene or butadiene copolymer rubber. Early in the polymerization, phase separation begins because of the immiscibility of the rubber within the polystyrene being formed and the depletion of the styrene phase. Grafting of polybutadiene with the polystyrene then takes place. Toughness, as well as other mechanical and rheological properties of HIPS, is strongly affected by the nature of the rubber phase. In this regard, some of the characteristics of the rubber which may be modified to control the overall HIPS performance include concentration, volume, particle size, grafting and crosslinking ability, molecular weight, viscosity, and vinyl content. More particularly, it is often desirable to have high vinyl content in the end segments. High vinyl polymers contain alkenyl groups pendant to the polymer backbone as opposed to a cis/trans configuration of alkenyl groups within the polymer backbone. High vinyl end segments are often desirable in a polymer because high vinyl configurations have a higher grafting efficiency than cis/trans configurations.
Polymers with high vinyl end segments have been produced by initiating polymerization and allowing the reaction to proceed to near completion. As the polymerization reaction approaches completion, additional monomer and a vinyl modifier are added. The final segment of the polymer thus has a higher vinyl content than the beginning segment. The vinyl concentration of the end segment can be controlled to levels as high as about 70%. However, if one wishes both ends of the polymer, or the ends of a star branched polymer, to have high vinyl content with the remaining segments having low vinyl content, this method will not work because coupling reactions result in high vinyl in the center of the polymer chain or star branched polymer. Making polymers with high vinyl end segments on all or some of the polymer chains and low vinyl content throughout the remainder of the polymer structure remains desirable.
In an exemplary embodiment of this invention, a polymer composition including at least two end segments which include less than 15% of the total polymer composition is provided. Each of the end segments has a vinyl content greater than about 45%, more preferably greater than about 50%. In contrast, the remainder of the polymer has a vinyl content of less than about 35%. This polymer composition is formed by producing, via living anionic polymerization in the presence of a vinyl modifier, a living prepolymer high vinyl initiator having a relatively low molecular weight (less than about 2500) and a high vinyl content (greater than about 45%, more preferably greater than about 50%). The living prepolymer high vinyl initiator is subsequently used to initiate a second living polymer chain containing less vinyl modifier than the living prepolymer high vinyl initiator. The second living polymer chain with a high vinyl content in the beginning segment is then coupled to at least one other living polymer chain with a high vinyl content in the beginning segment. This coupling results in a polymer chain with two or more ends having high vinyl content. Typically, the high vinyl ends will make up less than about 5% of the coupled polymers total weight.
In a further exemplary embodiment, a polymer with end segments of high vinyl concentration greater than about 45%, more preferably greater than about 50%, is provided with a linear, branched, or star-shape. The use of di- and multi-functional coupling agents can produce these alternate shapes.
In yet another exemplary embodiment, a polybutadiene polymer is made having end groups of greater than about 45% vinyl concentration, more preferably greater than about 50%, and a vinyl concentration less than about 35% throughout the remainder of the polymer.
In another exemplary embodiment, a styrene-butadiene block copolymer is formed with end segments having a vinyl content greater than about 45%, more preferably greater than about 50%.
The present invention relates to a coupled polymer composition including at least two end segments of high vinyl content prepared by living anionic polymerization, via batch or continuous polymerization. The polymer composition is provided with more than one high vinyl end segment by coupling two or more different polymer chains to each other. The polymer composition includes polymers which are linear, branched, or star-shaped, depending on the coupling agent used.
The polymer compositions of the present invention are formed by living anionic polymerization. Living polymerizations are polymerizations in which propagating centers do not undergo either termination or transfer. After 100% conversion is reached, additional polymerization takes place by adding more monomer to the reaction system. The added monomer is also polymerized quantitatively. Such polymerizations offer the potential for producing structures with defined end groups and block copolymers.
Three steps may be employed in the process of forming the present polymer compositions:
1. Preparation of a living prepolymer initiator having a high concentration of vinyl-configured butadiene units,
2. Preparation of polymers with high vinyl end segments utilizing the living prepolymer initiator, and
3. Coupling of the high vinyl end segment polymers.
To make this product in a continuous mode, the living prepolymer high vinyl initiator is made continuously in one reactor and fed to a second reactor where the bulk of the polymer is made. This continuous product can then be sent to a third device such as a reactor or mixer, preferably a reactor, where a coupling agent is introduced to react with the live polymer end. The coupling agent can be difunctional or higher.
Preparation of the living prepolymer high vinyl initiator is carried out by adding an initiator to a mixture of monomers in the presence of a vinyl modifier. Any initiator known in the art as suitable for the polymerization of conjugated dienes may be used. Preferred initiators include anionic initiators, and the most preferred are organo-lithium initiators. Preferred organo-lithium compounds have the formula RLix, wherein R represents a hydrocarbyl radical of about 1 to about 20, preferably about 2 to about 8 carbon atoms per R group and x is an integer from 1 to about 4. Typical R groups include aliphatic and cycloaliphatic groups. Specific examples of R groups for substitution in the above formulas include primary, secondary, and tertiary groups, such as n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, etc., and mixtures thereof A preferred initiator for the present invention is n-butyl lithium.
Specific examples of other suitable lithium initiators include p-tolyllithium, 4-phenylbutyl lithium, 4-butylcyclohexyl lithium, lithium dialkyl amines, lithium dialkyl phosphines, lithiumalkyl aryl phosphine, lithium diaryl phosphines and mixtures thereof
The amount of initiator used depends on the amount of monomer used and the molecular weights desired in the respective products. Under ideal conditions, where the polymerization system is free of impurities that can deactivate a portion of the initiator, the amount of initiator is determined by the amount of monomer being used for the initial block and the molecular weight desired in that block. Generally, in such polymerizations, each initiator molecule is responsible for the initiation of one polymer chain. Then for subsequent blocks, sufficient monomer is added in each case to form the desired molecular weight in the respective blocks in accordance with the number of atoms of initiator present in the polymer being formed. An exemplary amount of initiator in the present invention is between about 0.1% and about 15%.
Initiator is added to a mixture of monomers in a suitable hydrocarbon solvent in the presence of a vinyl-modifier. Preferred monomers are conjugated dienes. Suitable conjugated dienes include one or more of 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, and 1,3-pentadiene. Preferred conjugated dienes include 1,3-butadiene and/or isoprene.
Preferred solvents used in the preparation of the living prepolymer high vinyl initiator are hydrocarbon solvents such as pentane, hexane, heptane, octane, nonane, decane, cyclopentane, cyclohexane, cycloheptane, cyclooctane, and mixtures thereof.
A vinyl modifier is added to increase the 1,2-addition reaction of the diene monomer in the preparation of the living prepolymer high vinyl initiator. Such modifying agents are known in the art. These modifying agents may be employed in amounts generally ranging from about 1:100 to about 100:1 molar ratio of the modifier to anionic initiator. This yields a living prepolymer with molecular weight of about 500 to about 5,000. The 1,2-addition product can be increased from about the 5-15% range to as high as about 90% of the diene monomer units being incorporated into the living prepolymer high vinyl initiator. Preferred vinyl content of the living prepolymer high vinyl initiator is greater than about 45%, more preferably greater than about 50%.
Exemplary vinyl modifiers include one or more of hexamethylphosphoric acid triamide, N,N,Nxe2x80x2,Nxe2x80x2-tetramethylethylene idamine, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, tetrahydrofuran, 1,4-diazabicyclo [2.2.2] octane, diethyl ether, triethylamine, tri-n-butylamine, tri-n-butylphosphine, p-dioxane, 1,2-dimethoxy ethane, dimethyl ether, methyl ethyl ether, ethyl propyl ether, di-n-propyl ether, di-n-octyl ether, anisole, dibenzyl ether, diphenyl ether, dimethylethylamine, bis-oxalanyl propane, tri-n-propyl amine, trimethyl amine, triethyl amine, N,N-dimethyl aniline, N-ethylpiperidine, N-methyl-N-ethyl aniline, and N-methylmorpholine.
The number average molecular weight (Mn) of the living prepolymer high vinyl initiator is in the range of about 100 to 10,000, preferably in the range of about 250 to 5000, and most preferably in the range of about 500 to 2500.
Polymers with one high vinyl end segment are prepared in the second step of the polymerization process. These polymers are preferably linear, and can contain more than one monomer unit, in a random or block configuration. To form polymers with one high vinyl end segment, the living prepolymer high vinyl initiator is added to a mixture of monomers in a suitable solvent. The living prepolymer high vinyl initiator initiates the polymerization of the monomers by continuing living anionic polymerization. Preferred monomer units are conjugated dienes and vinyl-substituted aromatic hydrocarbons. The reactive system contains a lower concentration of vinyl modifier than was used to prepare the living prepolymer high vinyl initiator; therefore, the vinyl content of the subsequent polymer is of a relatively xe2x80x9clowxe2x80x9d vinyl content. For example, the vinyl content of this portion is less than 35%, preferably less than 30%.
Suitable conjugated dienes include one or more of 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, and 1,3-pentadiene. Preferred conjugated dienes are 1,3-butadiene and/or isoprene.
Suitable vinyl-substituted aromatic hydrocarbons include one or more of styrene, xcex1-methylstyrene, 1-vinyl naphthalene, 2-vinyl naphthalene, 1-xcex1-methy vinyl naphthalene, 2-xcex1-methyl vinyl naphthalene, as well as alkyl, cycloalkyl, aryl, alkaryl, and aralkyl derivatives thereof, in which the total number of carbon atoms in the combined hydrocarbon is generally not greater than 12, as well as any di- or tri-vinyl substituted aromatic hydrocarbons. One preferred vinyl-substituted aromatic hydrocarbon is styrene. If the di- or tri-vinyl substituted aromatic hydrocarbons are used, the resulting polymer has a branched and/or crosslinked structure.
Suitable solvents in this step include one or more of pentane, hexane, heptane, octane, nonane, decane, cyclopentane, cyclohexane, cycloheptane, cyclooctane.
The polymerization temperature can vary over a broad range from about xe2x88x9220xc2x0 C. to about 140xc2x0 C. The pressure used preferably is sufficient to maintain the fluidity of the substantially liquid phase under the conditions of the polymerization reaction.
The polymerization reaction is generally conducted for a time sufficient to obtain at least about 80% and preferably about 99% conversion. More specifically, the polymerization is preferably carried out until at least 90% of the charged monomer has been polymerized. The Mn of the uncoupled polymer is in the range of about 20,000 to 500,000, more preferably in the range of about 30,000 to 400,000, and most preferably in the range of about 40,000 to about 300,000.
The living prepolymer high vinyl end segment accounts for less than about 15% of the total weight of the polymer composition, preferably less than about 10% of the total weight, and most preferably less than about 5%. Therefore, the vinyl content of the living prepolymer high vinyl end segment has very little effect on the total vinyl content of the polymer composition. The preferred vinyl content of the total polymer composition is less than about 35%, and preferably less than about 30%.
The resultant polymer of the second step of the polymerization process is a living polymer with one high vinyl end segment. This living polymer is then coupled to other living polymers in the third step of the polymerization process. A wide variety of coupling agents can be used to couple the living polymers. A di-coupling agent will result in a linear polymer with both end segments having high vinyl content. A tri-coupling agent will result in a star-shaped polymer with all three end segments having high vinyl content, and a tetra-coupling agent will result in a star-shaped polymer with all four end segments having high vinyl content.
Suitable coupling agents include SnCl4, alkyl SiCl3, or an alkane dichloride of 1-10 carbon atoms, CO2, CCl4, SiCl4, PNCl2, divinylbenzene, butyl benzoate, CCl3COOEt, and mixtures thereof.
The appropriate amount of coupling agent is calculated based on the amount needed to react with the number of initiator atoms present in the polymer. For example, with a tetrafunctional coupling agent, such as SnCl4, one-fourth of the molecular weight of SnCl4 is calculated as required for each atom of initiator present. As a by-product, the initiator, e.g., Li+, is removed by reaction with chlorine and precipitated from the reaction solution as LiCl.
Other additives known in the art can be added. Stabilizers, antioxidants, conventional fillers, reinforcing agents, reinforcing resins, pigments, fragrances, and the like are examples of some such additives. Specific examples of useful antioxidants and stabilizers include 2-(2xe2x80x2-hydroxy-5xe2x80x2-methylphenyl) benzotriazole, nickel di-butyl-di-thiocarbamate, tris(nonylphenyl) phosphite, 2,6-di-t-butyl-4-methylphenol, and the like. Exemplary conventional fillers and pigments include silica, carbon black, titanium dioxide, iron oxide, and the like. These compounding ingredients are incorporated in suitable amounts depending upon the contemplated use of the product, preferably in the range of 1-350 parts by weight of additives or compounding ingredients per 100 parts by weight of the polymer composition.
A reinforcement may be defined as the material added to a resinous matrix to improve the strength of the polymer. Most reinforcing materials are inorganic or organic products of high molecular weight. Various examples include glass fibers, asbestos, boron fibers, carbon and graphite fibers, whiskers, quartz and silica fibers, ceramic fibers, metal fibers, natural organic fibers, and synthetic organic fibers. Other elastomers and resins are also useful to enhance specific properties like damping properties, adhesion, and processability.
The polymer compositions of the present invention are useful as additives in the preparation of high impact polystyrene. They are also useful when blended with polycarbonates or in acrylonitrile-butadiene-styrene terpolymers (ABS rubber). Impact strength of other systems is often improved when blended with ABS rubber, and high impact polystyrene blended with polyphenylene oxide has improved toughness and processability.